The present invention relates to an improved heat transfer fin and to a process for making and applying this fin to a refrigerant carrying tube, which has particular utility in refrigerant heat transfer.
In refrigeration applications, it is common to utilize a refrigerant-carrying tube to supply the means by which heat is removed from the chamber or areas to be cooled. Ordinarily, the heat removal is accomplished by forced convection between two separated fluids. For example, in household refrigeration applications such as refrigerators, the two separated fluids would be [1] a refrigerant contained within a cooling tube and [2] air flowing across the refrigerant-carrying tube to assist in transferring heat to or from the tube wall as imparted by the heat of vaporization or condensation of the refrigerant within the tube. In the applications just mentioned, the refrigerant carrying tubes are usually provided either as a condenser or an evaporator.
In such forced convection applications, it is common practice to provide a balance between the amount of heat transfer surface area and the heat transfer coefficients at the respective surfaces. Ordinarily this balance is maintained in an inverse relation. Thus, where the particular fluid has a relatively low heat transfer coefficient, a greater amount of exposed heat transfer surface is generally provided. In addition, the practitioner seeks an economic balance between the amount and structure of the exposed heat transfer surface area considering the heat transfer coefficients of the fluids involved. As an example, in a standard refrigeration application, the refrigerant has a significantly greater ability to transfer heat to the tube in which it is carried than does the air which flows thereacross to remove the heat transferred to the tube by the refrigerant. As a result, it is an accepted practice in the refrigeration art to substantially increase the surface area provided on the outside, or air side, of the tube to balance the ability of the refrigerant to supply heat to the inside of the tube.
Most often, the increased surface area provided on the air side of the refrigerant carrying tube is provided in the form of some sort of extended cooling surface or fin extending from the tube. Many types of finned tubing are commercially available for use in refrigerant-to-air heat exchangers (both evaporators and condensers). One type of extended surface fin is the type of strip fin known as a "spine fin" as disclosed in my prior U.S. Pat. No. 2,983,300. Other types of extended surface fins are disclosed in U.S. Pat. No. 4,143,710 issued to LaPorte et al. These latter fins are complex geometric shapes, which are difficult to fabricate and have a higher degree of wasted material in relation to the heat transfer capacity provided. The spine fin has a disadvantage in that it is mechanically weak and has a low resistance to bending and compressive forces. Therefore, to permit practical utilization of the spine fin, in use the spine fins are spaced or bunched very closely on the refrigerant tube.
The spine fins and geometric fins have been used successfully for many years to increase the surface area on the air side of refrigerant carrying tubes in home air conditioning units (i.e., the evaporator), where the operating temperature of the air flowing across the air side of the tube is above the freezing point of water. Heretofore, however, cooling fins similar to the spine fin and the geometric fin have not been successful in environments where the air temperatures are below freezing, primarily for two reasons [1] because the moisture in the freezing air condenses out and forms a "frost bridge" between the closely spaced spine fins or portions of the geometric fins, which materially inhibits the air flow across and between the spine fins, which in turn reduces the heat transfer capability; and [2] if the fins are spaced far enough apart to prevent frost bridging, the resulting structure is too mechanically weak to permit practical fabrication on an industrial scale.
Mechanisms through which frost accumulates on evaporation fins are understood (cf The Frost Formation on Parallel Plates at Very Low Temperatures in a Humid Stream by M. C. Chuang, ASME paper 76-WA/HT-60, presented at the ASME Winter Annual Meeting, New York, N.Y., Dec. 5, 1976), and fin structures have been developed for frosting applications. Typically, these fins are plates that extend at right angles across a number of tubes. The plates are spaced relatively far apart, typically 4 or 5 fins per inch, to reduce frost bridging. Their performance is adequate, but markedly inferior to the smaller spine fins from a heat exchange standpoint. Thus, smaller fins with sufficient mechanical strength to allow them to be placed far enough apart to effectively reduce frost bridging are desireable. Of course, since the fins are intended for frosting applications such as refrigerator evaporators, the fins should also be designed to avoid or minimize frost accumulation.